Hitherto, as the element member of semiconductor device, photosensitive device for use in electrophotography, image input line sensor, image pickup device, or other optical devices, there have been proposed a number of amorphous semiconductor films, for instance, an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atom or/and halogen atom such as fluorine atom or chlorine atom [hereinafter referred to as "A-Si(H,X)")]. Some of such films have been put to practical use.
It is known that such deposited films may be obtained by means of a high frequency glow discharge decomposition process (hereinafter referred to as "rf-PCVD method"), that is, a process of forming a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum by decomposing a raw material gas with a glow discharge using a high frequency energy (13.56 MHz). And there have been proposed various apparatuses for practicing said method.
However, there are unsolved problems for the rf-PCVD method such that the utilization efficiency of a raw material gas is low and the deposition rate of a film is slow.
In recent years, as a means to solve the problems in the rf-PCVD method, the public attention has been forcused on a glow discharge decomposition method using a microwave energy (hereinafter referred to as "MW-PCVD method") at industrial level.
Along with this, there have been proposed various apparatuses for practicing the MW-PCVD method.
One representative apparatus for practicing such MW-PCVD process is such that has a structure as shown in a schematic perspective drawing of FIG. 4.
In FIG. 4, there are shown a whole reaction chamber 401, a substantially enclosed deposition chamber 402, a microwave transmissive window 403 which is made of dielectric material such as alumina ceramics or quartz, a waveguide 404 which transmits a microwave 412, a microwave power source 405 which generates said microwave 412, an exhaust pipe 406 being connected through an exhaust valve (not shown) to an exhaust apparatus (not shown), a ring-shaped gas feed pipe 407 being connected through a valve to gas reservoirs (not shown), gas liberation holes 407', a substrate holder 408, a substrate 409 onto which a deposited film is to be formed, an electric heater 410 for heating the substrate and a plasma generation space 411.
In general, the deposition chamber 402 has a cavity resonant structure so as to resonate with the oscillating frequency of the microwave power source 405 since the deposition chamber 402 self-excites to initiate a discharge without a discharge trigger.
The film forming operation in the apparatus shown in FIG. 4 is carried out in the following way.
That is, the air in the deposition chamber 402 is evacuated by opening the main valve of the exhaust pipe 406 to bring about the space 411 of the deposition the chamber to a predetermined vacuum. And the heater 410 installed in the substrate holder 408 is actuated to uniformly heat the substrate 409 to a predetermined temperature and it is kept at this temperature.
Then, raw material gases, for instance, silane gas and hydrogen gas etc. in the case of forming an amorphous silicon deposited film, are introduced into the deposition chamber 402 through the gas feed pipe 407 and its gas liberation holes 407'.
At the same time, microwave 412 having a frequency of more than 500 MHz, preferably of 2.45 GHz is caused by the microwave power source 405, which is successively introduced into the deposition chamber 402 through the wave guide 404 and the microwave transmissive window 403. The raw material gases thus introduced into the deposition chamber 402 are excited and dissociated by an energy of the microwave to generate neutral radical particles, ion particles, electrons and the like and to cause chemical reactions among them resulting in formation of a deposited film on the surface of the substrate 409.
By the way, in the known MW-PCVD apparatus, the microwave transmissive window must firstly serve as a means to introduce a microwave into the deposition chamber.
Then, it must serve also as another means to maintain the inside of the deposition chamber in a vacuumed state and at the same time, to maintain the gaseous atmosphere composed of the introduced raw material gases in the deposition chamber. In view of this, the microwave transmissive window is connected through a vacuum sealing O-ring to the deposition chamber. Details of this situation are as shown in FIG. 2, which is a schematic partial cross-sectional view for the part of said vacuum sealing O-ring through which the microwave transmissive window being connected to the deposition chamber. In FIG. 2, there are shown circumferential wall 201 of the deposition chamber, microwave transmissive window 202, vacuum sealing O-ring 203, microwave transmissive window supporting wall 204, microwave plasma space (deposition space) 205 and microwave 206.
And in such know MW-PCVD apparatus, in order to ensure the vacuum sealing of the microwave transmissive window 202 with the deposition chamber through the vacuum sealing O-ring 203, the surface of the microwave transmissive window facing the deposition space 205 is required to be in a well grinded state.
However, there are unsolved problems for such known MW-PCVD apparatus using a desirable microwave transmissive window having a well grinded surface that for example in the case of forming a deposited film composed of a A-Si(H,X) material on a substrate, said A-Si(H,X) material becomes deposited also on the surface of the microwave transmissive window facing the deposition space (hereinafter "the inner surface of the microwave transmissive window") and along with this, the microwave transmissive window becomes heated with absorption of a microwave energy and also with a plasma heat. That is, the resulting A-Si(H,X) deposited film on the inner surface of the microwave transmissive window becomes cristalized and low resistant due to elevation of the temperature for the microwave transmissive window, and along with the increase in the resulting A-Si(H,X) film on the inner surface of the microwave transmissive window and also along with the elevation of the temperature for the microwave transmissive window, the reflection of a microwave at the microwave transmissive window increases accordingly. Because of this, the effective power of a microwave to be introduced into the deposition space decreases so that the decomposition rate of a raw material gas in the deposition space and the film deposition rate of a deposited film composed of a A-Si(H,X) material onto a substrate becomes lowered.
In view of the above, for the known MW-PCVD apparatus, there are unsolved problems that it is difficult to continuously form a deposited film for a long period of time and it is necessitated to often change the microwave transmissive window.